My Gencon game this year had a theme of Zombies/Cthulhu and wanted to put in a Chthonian for the "Big Bad" at the end that the players had to fight but I couldn't find a mini to match that was affordable. I saw that Reaper Miniatures was selling a Purple Worm in their "Bones" Line that looked like a Chthonian but it needed some tentacles. So I used my Google-Fu and found this site on how to make tentacles.
http://miniaturewargameconversions.blogspot.com/2012/03/luckyno5-sculpting-octopus-like.html
With that tutorial and some drilling, here's what I came up with. It was surprisingly easy!

I also want to give a shout out to Reality Blurs. I got the stats from their Realms of Cthulhu. Great book by the way!

Side view of the Chthonian. With a human sized zombie next to it for size comparison.

Another side view of the Chthonian. With a human sized zombie next to it for size comparison